I'd suggest making a custom container. Find a Rubbermaid tub like a cat litterbox or similar, with a footprint such that it just fits into the smaller tank without turning. Cut one end off. Lower the water level as suggested until you can stick the container in so that it just reaches the top of the water level, then you can easily scoop the fish up by moving the container down the length of the tank. That way the bottom of the box completely supports the fish and it won't get hurt. Do use gloves since you may have to keep it from sliding out (extra hands would be good here). I assume we're not talking about carrying it a huge distance here?
As far as acclimation, I think I would lower the temperature slowly until both tanks were just room temperature, no heaters. Then you know they are the same. Plecos can tolerate a wide enough range that room temperature shouldn't be a problem unless there are other less tolerant fish involved. For chemical parameters, Rob28's suggestion seems best. A series of water changes where you just mix the water from the two tanks, or empty the smaller and refill it with water from the larger, while refilling the larger from the tap, will equalize everything so there would be no shock.
