not sure if it is what you would consider a lot for a feeding, but I have been growing my own phyto for years- Usually feed a couple cups 4 times a week to a 58G tank. I grow it super dark- totally do not follow the "rules for culturing it. I am fairly certain I derived a strain that is bullet proof and near impossible to crash.
Don't feed clumps- they don't re-disperse well. If you are culturing it correctly, there shouldn't be issues with ammonia. Filtering it really isn't an option. Yes, you would need to vacumm line it, it would immediately clog up the filter, and probably rupture the algae cells. Centrifugation would be a good option, but not something most of us have sitting around the house.