I have a small rig on the side running with 2 1070, I'm building a new 1080ti rig.
But can i add one 1080ti to rig with 1070 it will work fine together?
Should I mine the same thing with al three or can I arrange 1080 for the others too without much headache?
If it's possible, yes (the others have already answered you on that)
If you should mine the same algo or different for the 1070s and the 1080ti, that depends on what you mine.
If you mine a specific coin and hold it, then maybe you should mine with all cards but it depends on the algo. If you mine whatever coin is more profitable and swap it, then these 2 cards don't excel on the same algos and you might want to setup two instances of NemosMiner or a similar software.
For example a 1070 is "mh... okay" for ETH but the 1080ti is terrible. On the X17 they're pretty equal and I believe the 1080ti performs better on X16r.
It's not hard to set every card on a different miner and algo, it depends on your knowledge and your nerves!