Sure, you can have more civs - and you don't need to pay FPS to increase world size to do this.
I've put 206 civs in pocket world, and that's far from maximum.
However, I think it is a meh solution, because you'll likely not get elves, humans and goblins all coming from a big major site with that as closest location (at least 3k sentient pop for humans and elves and 10k for goblins) at the same time, while also being their only available target.
Simple worldgens are not that good for this because the smaller retreats, halmets and pits tend to get placed in a ring around the major site. Pre-embark two week crawling FPS aside, I cringe whenever I see a medium worldgen and see no sites I'd consider properly filling anyone's request in worldgen thread.
However, if you're not painting I suppose they can enable larger geographical distance - the snaking patterns required otherwise don't really appear unless forced.
That said, of course medium worldgens have bigger chances of RNG not eating given civs - that's kinda given with medium worldgen being 55,8 pocket worlds - for an accurate comparison, should look through as many pocket worlds with proportional settings for each medium world.
However, I think there might be something of a RNG solution:
One thing I recently found in Succession World is that megabeasts can cross the ocean to come to your fortress. I'll have to ask, do you have them also cross from other side of the world, i.e. from artics while you're in tropics?
Anyway - in worldgen, they cannot do this. Tested it with single elven island separated by 1-wide sea gap VS without sea gap.
So if you're not so picky about available biomes you can do triangle like PatrickLundall's PALU2, expect looking something like this instead:
π~VVVVV¶~
~~~~~~~~~
~V~ " " " " " " . . .
~V~ " Here be dragons
~V~ "
~V~ " .
~V~ . .
~+~ . .
~~~ . . . . Ω
The main issue here is the required 7-tile separation for building new major sites - human town and elven retreat, thus leaving 13 undesired squares VS 240 desired (in pocket world) onto which megabeasts can spawn - an unattractive ratio of 18 to 1. Well, increasing the world size to 33x33 will more than quadruple the ratio, so there's that.