Both can be frost seeded any time. Both r “heavy” hard seeds that will make it to soil as long as it’s not chalked full of trash. The freezing & thawing of soil will create a very desireable seed to soil contact that’s needed. Perfect timing now & in the next few weeks. Switchgrass- id do this NOW! U need stratification to take place on switch seed (freezing & thawing to break down hard seed coating so seed will germinate this year). Clover u have a little more time. Do it all now though & ur fine.