Assuming they do a fine job installing new flooring (Pergo-style, if it matters) should the installers be tipped? If so, how much?
I have never tipped a floor installer.
Buy them a pizza and some beer. Tell them you think they did a great job, that you’re really happy with their work.
Homemade cookies work really well, too.
If you get the pizza delivered, tip the delivery guy. 
As a replacement window and door installer for a large chain, I’ve been tipped on occasion. Nothing in our contract prohibits it, but it’s not expected at all. We recylce the metals out of the tear out and clean up, which can sometimes add up to a nice chunk of change. Floor installers don’t have that neat little perk.
Besides, we are very well paid by the chain.
As for beer as a tip, I never drink alcohol on the job. Unprofessional and unsafe. IMHO, YMMV
If you want to reward a worker for a job well done, do whatever you’re moved to do. If they do refuse, it’s quite possibly a contract stipulation. (The contract between the installer and the store, not the store and the customer.)