Can I install lights on a 20 amp circuit that has receptacles already wired?
According to 2011 National Electrical Code, there is no problem mixing lighting and receptacles on a 15 or 20 ampere circuit.

2011 National Electrical Code
Article 210 Branch Circuits
210.23 Permissible Loads.
(A) 15- and 20-Ampere Branch Circuits. A 15- or 20-ampere branch circuit shall be permitted to supply lighting units or other
utilization equipment, or a combination of both, and shall comply with
210.23(A)(1) and (A)(2).

For some reasons why folks tend to avoid this in practice, take a look at my answer to this other question.