Can I use frozen fruit in this pie?

Yes, frozen strawberries and frozen rhubarb can be used for this! Make sure both are completely thawed and drained well so you avoid a watery pie. You will likely still need to slice both the strawberries and rhubarb.

Do I have to use a lattice top on this pie?

Lattices can be finicky if you aren't practiced, so it's totally fine to just use a regular pie crust! Drape the crust over the filling. Fold the bottom overhang over the edge, and crimp all the way around to seal. Use a paring knife to cut slits into the center of the pie to allow steam to escape.