Packing Question

I think there's virtually no chance you will need shorts. There's a non-zero possibility you might have a day or two when it is warm enough you wouldn't be too cold in shorts. But that might well be the situation for only a few hours in the middle of the day. I have no useful input on the swimsuit question.

Caveat: I do not live in Scotland. I am answering on the basis of 26 days' experience traveling around the county (not including the far north) in 2019.

It depends what you plan on doing and your affinity for wearing shorts. (I live in New England and I know people--mostly men--who make a point of wearing shorts any time the temperature is up above about 50 Fahrenheit.) I don't think there's any time of year in Scotland when you'd really need shorts, but you could wear them on at least some days in June. My observation is most Scottish people peel off upper layers when it gets warm, but keep the jeans and boots on, so you'd see young women in a T-shirt or tank top with jeans and boots on a warm day. And a cardigan and/or windbreaker always within reach.

If you want to swim in the fairy pools or the ocean, or plan to stay in a hotel with a pool, then by all means bring a swimsuit. If in doubt, better to have it handy than not!

You can see actual weather records by place and past years/dates on the Weather Underground website. For example, here is June 2020 in Edinburgh. https://www.wunderground.com/history/monthly/gb/edinburgh/EGPH

Oddly, it shows zero precipitation. Something must be out of whack with that.

https://www.scotland.org/about-scotland/weather

With summer time high in the mid 60sF shorts seem unnecessary.

Only place I can imagine a need/use for a swim suit would be the hotel pool

Temperature averages for June

https://en.climate-data.org/europe/united-kingdom/scotland-257/r/june-6/#climate-table-month

It can get hot enough for shorts... They don't take up much space, so may be worth packing a pair. Alternatively what about a pair of cargo trousers that have zip off legs? Lots of companies sell them on the internet...

A bespoke weather forecast for Scotland

https://www.taps-aff.co.uk/

Ramblin on, that is ace !!!

These days, average weather info isn’t as useful as it would have been years ago a the weather is now so unpredictable.

A couple of days ago, I was in shorts and a T-shirt as it was 18 degrees. Average weather forecasts tell me it should be 9 degrees and 12 days of rain in March (not had any rain for over 3 weeks).

The weather in Scotland is going to vary significantly depending on where you are visiting. I have been in June and had 7 degrees daytime temperature near Aberdeen. Last June in North Berwick near Edinburgh, we were in shorts and T-shirts as it was 20 degrees. The sea is cold, so unless you have a hotel with an indoor pool or you are very hardy, I would not bother bringing a costume.

People around where i live wear shorts all the year round, especially postmen, even when its below freezing
Myself. its got to be hot before even thinking about shorts

PS its 8 C and trying to rain. Next door neighbours's gardener/handyman just turned up on his bicycle. He is wearing shorts!

Given that you are in Arizona, I imagine you might find it a 'bit nippy' for shorts in Scotland in June. However, many locals (including myself) are keen to bare our pale legs at any opportunity in the hope of some much needed vitamin D :)

Zip off legs might be the answer.

As for a cozzie (Swimsuit) - well it depends if you fancy a dip in some of the magnificent, mountain fed pools that can be found on any number of wonderful hikes.

10 degrees is when the shorts come out for me, but I live a lot further north than you.

I lived in Scotland for many years and didn't even own a pair of shorts.
I don't think you will need them at all.
Maybe a lighter pair of long pants, and layers.
I'm the one who packs long underwear for every trip to Scotland, regardless of the time of year.

Edit:
I see you are from Arizona, so you may not be used to the damp cool weather that Scotland often has.
Pack your layers for Scotland in June!

This is a good topic, and when I think about it, my upcoming trip to Scotland in June is the most challenging trip I can remember planning from a packing perspective. It's currently 10C and I'm sitting on my west facing deck in shorts and sweatshirt but also a blanket nearby when the sun goes behind the trees. In two hours it will be too cool for the deck this time of year. It's easy to change clothes at home to adjust, but trying to figure out the right combination of layers to fit in a small suitcase is a new experience.

If you hit on a hot spell you can always buy a pair or a kilt

The answer is to pretend you are British and bring a bigger bag that fits everything in.

From a sunny 18 degrees at the weekend to 2 degrees and snow at present.

And I was in Glasgow at Easter a crew years ago, and it was about 23C and definitely “tops off” for all the lads and most of the lassies as well.

I was in Scotland in August a few years ago. I didn't wear shorts while I was out and about (wasn't warm enough), but I was glad to have a pair to wear/lounge around in my hotel room. After walking around all day, I was a little hot and sweaty. There was no AC in the room, so I was glad to have my shorts.
Unless you know for sure you're going to be swimming, I wouldn't bother with the swimsuit.

At my wife's suggestion, I was the one who was filling our rental car on the Isle of Skye on the Fourth of July. The two or three other guys filling their cars (all Scots I assume) were wearing shorts and t-shirts. It was sleeting sideways.

You don't "need" either. In the past, I've done walking holidays so have always had convertible pants which have solved the problem for me on the VERY few days when I've decided it's warm enough to merit them. I stopped bringing a bathing suit, but I have one friend who loves to swim in the sea. Yes, it's cold, but that never seems to stop her! The sea close to ten degrees colder than it is off Cape Cod in June. :)