Slovenia and Croatia Rain gear in early May

Hard to predict the future. Check the extended weather forecast shortly before you leave. I would at least take a packable rain jacket. If you find that you need other gear, you can always purchase it there.

The rain pants always intrigue me. I don’t own a pair and have never brought a pair on any vacation.
I would think they are thin and lightweight and could squish down in your suitcase. If it would give you peace of mind then bring them.
Anyway, I have probably been to Croatia every month except January and February. Never needed them.

I own rain pants but have never taken them to Europe. I hate the combination of cold + wet, so I'd take them on a trip with a significant percentage of time in cold-weather destinations, which could include spring/summer/fall hiking at altitude. But I don't hike at any elevation, and my trips fall mostly between May and early October.

I will say that probably the wettest I've ever been in my life (I'm 73) was in Zagreb, in early October 2015. It was a deluge of Biblical proportions. And fall was moving in, so the temperature had dropped a good bit. Not pleasant, but it wouldn't have been worth taking rain pants on that trip even if I had known that would happen.

What I swear by now--after a tip on this forum--is 96%-97% nylon slacks of the type sold by Eddie Bauer, Colombia, PrAna and no doubt others. They aren't waterproof, but they do a pretty good job of shedding rain, which is helpful if you're traveling with a rain jacket rather than a full-length raincoat. When the day is chilly, I wear long johns (usually merino wool) under the slacks; that reduces the clammy feeling quite a bit if I get caught in heavy rain. But again, I don't hike; I just walk a lot in cities. I've never gone back to my hotel in the middle of the day to change clothes, but it would usually be possible for me to do so, if I wanted to. That's different from being out in the country and getting soaked to the skin.

I live in a famously rainy city. And I do have good rain pants. I have brought them to Europe a couple times, but it depends very much where I'm going and what I expect to do there. I would not bring them to Slovenia and Croatia in early May, UNLESS you plan to do some moderately serious hiking, and your plans are not flexible.

I've been to Slovenia and Croatia in May (mid-May). We got a couple days when it rained like hell, though not all day long (there were intense thunderstorms in the mountains when we were in Bled, and snow forecast in the passes; rained hard overnight but next day was delightful). If it had been raining hard during the days, we would have stayed inside and skipped hiking, or just dashed to and fro in our light rain jackets that we always have. So bottom line, for me at least, I would leave the rain pants at home for a trip to Slovenia and Croatia in May (unless your plans included hiking and you intended to stick to that plan regardless of the weather). A light (but waterproof) rain jacket should be adequate unless you are determined to be outside all day, even if it's pouring.

I did use my rain pants (and they may have saved my life) on hikes in the Faroe Islands (that was in July, mid-summer). Rained hard for days on that trip. I think the chance of days-long rain in Slovenia and Croatia in May are low enough that I'd have a hard time justifying the rain pants on that trip. YMMV.