If you're talking about hybridization, please listen to
@DoubleTake and don't. I have a peachface/eyering hybrid gal, and I love her dearly, but she's a real mess mentally. Eyering mixes are usually better (personata/fischerii) but are also not advised due to the muddying of genetics and how different the two species actually are when you look at them.
As far as keeping mixed species, I also wouldn't advise it. My fischers all get along quite well, but my black masks are much larger and could easily bully my fischers. Thankfully, they both are calmer birds and seem to mix well enough with the fischers and fischer/mask hybrids I have.
My peachface tolerates my black masks and fischers, but only in small doses. They don't really share the same body language or sound the same, so they have difficulties communicating at time. There are enough similarities that they can, but he prefers to stay with my cockatiel over the other lovebirds.
If you're doing this, you also have to be aware that not all lovebirds will want to share a cage or aviary. If you have more than one, you should have more than one cage. Yen (My PF/Eyering) will not share her cage with any other bird and will attack anyone who comes near.
Females can also be very territorial of their cages, mates and people. I've caught Yen and Triss puffing up in anger if I pet one of them and not the other. It's a good thing I have two hands, or I'd likely have a fight on my hands.
I even had an issue with my two boys where one threw another off a shelf due to hormones and general agitation.
They have been known to kill other lovebirds, even their own mates, so you have to be very careful with what you're doing.