20gals is not big enough for a common plec, although you are doing him a big favor by taking him on into a bigger tank, your tank is already too small for a common plec. They need at least 50gals with lots of surface area, and thats still a tad on the small side. Are you thinking of upgrading your tank soon? If your guppys are adults then your tank is already overstocked without the plec, and you will need a bigger tank if you plan to keep the plec for the rest of his life(which could be 20yrs+).
Other than that, make sure you have test kits for ammonia, nitrates and nitrites because plecs are big waste producers and your tank is already very stocked and adding him could be too much for your filter to handle so make sure you test your water regualy and do regular water changes with dechlorinator.
Common plecs also need meat in their diet as they grow, you should try to feed your plec a prawn or small peice of fish meat(raw) once a week and do a water change the day after you have fed the prawn as so to prevent water quality issues from the high protein food.
Also provide a large cave/retreat area where the plec can rest during the day out of the glare of the tank lights as common plecs are largely nocturnal, bogwood is also beneficial to all plecs as the tanins(the stuff that makes the water go tea colored if you don't pre soak the wood enough) have anti bacterial qualitys in them good for plecs.
Common plecs do not destroy plants although they will eat ones that have algae on them or are going rotton, what they can do though is accidentially pull up plants when patrolling the tank so make sure any live plants you have are securely rooted and there are open areas in the tank where the plec can swim in untroubled by things in its way. Cucumber is also a healthy alternative to algae wafers as it contains alot of vitamin C and you can feed your plec this also if you like, don't give it more than it can eat though as rotting food makes an ideal home for fish deseases like columnaris to grow and fester in and food does not take long to go bad in fish tank water remember.