Do screen covers and substrate trays count as complete reptile enclosures?
Screen covers and bottom trays are accessories that pair with a tank or cage. They are not standalone habitats. They can improve ventilation or make cleaning and substrate management easier, but you still need a secure enclosure that provides room for basking, hiding, climbing, and proper temperature control. Use these items to complete a setup, not replace it.
Should I choose a reptile enclosure by gallon rating or by actual dimensions?
Go by the enclosure's length, width, and height rather than just a gallon label. Dimensions tell you if there is enough floor space and vertical room to create a temperature gradient and to add hides, branches, and basking areas. Gallon ratings are a rough volume estimate and can be misleading when two enclosures have very different footprints.