I think basic windsurfing skills in a natural progression to foiling because once your up on the foil its a whole new learning curve and windsurfing doesn't apply so much. What I mean by that.... (for reference, I started windsurfing in 75', and am a new foiler. )
One of my mistakes was I was leaning out trying to load the rig up like I would normally blast on a fin board. Foiling requires a lot more finesse. Micro weight shifts, constant minor sail trim etc. To keep flying straight. A much more upright stance to de-power the rig when gusts hit. This is from a beginner on a free ride rig (SS 114, I76 wing). The formula type racing rig, seems those guys hang way out using massive boards and sails. 120L board should be pretty ideal to get started. I'm 140 lbs, and took a while to slog well on 114L board. There is a lot more "tuning" involved with foiling, so some patience is required. I started in late April and got things pretty well sorted out now for straight line foiling and carving around (fun), but still suck at jibes

. I do get in a lot of water time though as I live on the waterfront and all my gear is rigged on my dock ready to go. 2 weeks was spent healing up from kicking the foil water starting, so be very aware of where the foil is. No need to water start.