I was wondering what Artoban had used for bases with his, and so called up this thread. Looks like he used the round ones that come with the kits, and not the square ones. As it turns out, the square one's are 25mm instead of the WFB size of 20mm, so now I am still undecided about what I will do. 
You could put your 25-30mm human-sized figures on round 20mm bases. You can then put them on trays where the side of individual square base is assumed to be 20mm. And you can get those 25mm rings to but around your 20mm bases, to upgrade them to 25mm round bases where the game absolutely needs those.
As it turns out, I remeasured the plastic bases that come with the Forgotten Worlds figures, and either size provided fits with the various other medieval figures I am using for Lion Rampant, and thus both types of bases easily fit for Dragon Rampant. Now it is just a matter of what I'd prefer to do.
Or you can move to play Oathmark, where ranked-up infantry is always on 25mm bases, which can be round or square, depending on what kind of movement trays you build. Or move to play Kings of War, where the number of models on the unit tray is not dictated by the game.
But for friendly games, I would assume the base sizes aren't that important really.
Yep, I have figures based on both the square and round bases, and it really doesn't matter for the Lion Rampant and Dragon Rampant games, plus the other rule sets in the same series.
I now have LR, DR, Pikemen's Lament, Rebels & Patriots, and The Men Who Would Be Kings.
Personally, I'm putting my historical/fantasy figures on 25mm round bases, and then playing rank-and-file games where individual base size is not that important. Just use trays that fit the game, and organize the figures on the tray as best. It's a bit problematic for med and hvy infantry, who are supposed to form tight units like pike blocks or shield walls.
But this approach is a luxury allowed by hundreds and hundreds of traditionally based Warhammer models, which I can use in that kind of games.
In the end, miniature game hobbyists tend to eventually build & paint so many models that you can eventually have several differently based armies for different games.
With the whole Rampant series of rules, I cross my figures from fantasy to historical, and care much less about the base sizes at this point.
By the time this Covid crud get done, I'll have multiple armies I can bring on a trip to where ever. I have a brother in Wisconsisn, and would probably try to meet up with S.O.F. there as well. Plus looking forward to connecting someday with Artoban and Gankom in Canada, would even consider bringing an army to Europe to meet up with some W-E people on future trips there. In fact, there is a historically aimed guy in France where he and his wife have a bed and breakfast, and they provide historical tours in the morning or during the day, with wargames in the afternoon or at night. These all sound like good vacations to me.
