I don't think what you are asking will work. The simple solution would be to use what you have. Your lights and seed trays, if you use standard seed trays, will only hang over the sides of the shelf a few inches on each side. You should be able to fit two fixtures on each shelf. Just make sure you have enough light to cover the seed trays you use.

I did this with T8 fixtures (but the ballast was designed to
light either size...some T5 ballasts are made the same way).

I don't know about Lowes (the stores here don't sell T5), but
Home Depot sells 3' T5 fixtures and they are less expensive
than the 4' T5 ones.