I haven't had any experience with the Bolivians, but I've kept GBR, agassizii and and cauduoides.
In my experience the cauduoides was a bit more aggressive than the agassizii.
In both cases I have found a lot better results with Harems vs pairs. I typically have them in 20 Longs and find that the best groups start with about 6-8 fish with no more than 2 males. From there, I more or less let them sort it out. The final results are normally around 3-4 females and 1-2 males with decent size territories and enough rivals to keep them from picking on a single fish to death, although I typically lose 1-2 of the weaker fish along the way to reaching this equilibrium.
Everytime I have tried to keep just a pair or a trio, inevitably all but one fish dies. Typically, one female kills the other female, and then the male kills the last remaining female. Contrary to intuition, my aggression problems decrease the more fish I have, because it forces them to have to spread out their anger. Makes sense when you think about it.
I doubt they will do too much to the adult Endlers, because they are typically only really mean to their own kind. They definitely get testy with everybody after spawning, but in a 30 gal it shouldn't be too much of an issue. If you have concerns, throwing more Apistos into the tank will help solve this issue, because they will be far more concerned with their own to mess with the Endlers. Also a bit of floater coverage will help give the Endlers, adults and fry, a place to get out of the way and hide. Also, as you have noted, Apistos will be hard on the fry.
In terms of personality, I feel like they differ some from fish to fish, but generally all of the Apistos are going to be more aggressive than Rams (Certainly the case for GBR). Between the cauduoides and agassizii, I never really noticed any significant or regular personality differences other than the standard individual fish differences. No real trend between the species.
I also found both Apistos species to be about the same in hardiness. But they are going to slightly more difficult than rams are. I wouldn't put Apistos (at least the species mentioned) on a whole 'nother level than GBR, but given that Bolivians are hardier than GBRs and GBRs are hardier than Apistos, there is prolly a noticeable difference in the ease of care.
Bottomline, your biggest issue getting into the Apistos is going to be finding the balance on the aggression issue. Once that works itself out, I find that the Apistos are not any more difficult to maintain than other dwarfs. And in terms of solving that issue, I have found that it is important to have 3 things: 1) hardscape that lends itself to breaking up lines of sight and territories, 2) plants, the more the better, and 3) more, not less, Apistos with a female heavy ratio.
I would also add that despite the extra challenges Apistos create, the results, once the tank has settled in, are amazing and well worth the effort. Apistos are incredibly beautiful, rewarding, and entertaining fish to keep.