As long as you have your equipment pad poured large enough to handle a possible heater, and your plumber knows that a heater may be in the future and plumbs accordingly, there's no reason the solar can't go first.

I hope you arrived at a satisfactory solution 15 years ago. I'll add a comment in case someone else ends up here as I did. I made a passive solar pool heater as an experiment. I used a submersible aquarium pump in the pool deep end to run water through 100' of coiled ABS mounted to a 4 x 8 sheet of black painted plywood. The water going in was 66 - 68 F, The water coming out was 125 F. during a sunny day. Because of thermal stratification (pre-circulation) the warm layer of water was the top 18 " of the pool surface. Maybe this will be useful to someone else.