As others have said, any formation flying requires training and skills. Also, by law in the USA, a briefing is required. If you were formation trained and qualified in the military, take some time to learn the procedures of the group that you may choose to fly with. Some of the signals and cues are bound to be different. There are two organizations in the US that can issue civilian certifications that allow the pilot to fly in FAA waivered airspace (airshows). I highly recommend training with either the FFI or FAST. Nominally, FAST certifies the warbird community, and FFI covers the civilian aircraft. That is not strictly divided. I have flow with both, and maintain certification with FAST through their signatory agency, The Redstar Pilots Association (RPA). The instruction through Redstar is IMO the best available. Most of the instructors are ex- or current military with thousands of hours training for formation in difficult conditions... such as in-air refueling - one of the worst dissimilar aircraft situations you could imagine. The RPA standards are high. No one gets the patch unless they can fly to the standards. It is also an inclusive and fun-loving group that puts on some of the best events. Oh... I've also had the fantastic opportunity to fly formation with a P-51D, F-4F Wildcat, F-4U Corsair, and other beautiful warbirds, all courtesy of the RPA.
Dissimilar formation is more difficult, but manageable through briefing and practice. I fly a Lancair 235/320. It's great to fly with some of my formation buddies here on the LML, but I also fly with Yaks, CJs, T-34s, Glasairs, Swift, Skybolt, Grumman Yankee, Bonanzas, etc. Compared to all those except the Lancair/Glasair, my aircraft is much lower drag. The result is that I will overshoot on a downline if the other aircraft do not keep the power on. If the briefed mission calls for steep descents in close formation, I may drop the landing gear about 2" and lower the flaps an inch. For normal parade flights with slower aircraft, I leave the flaps down about an inch. Otherwise the wing is on the edge of the laminar drag bucket and requires much more attention to maintain stable airspeeds below 140 kias. Even with the extra drag, I typically show 10-14" MAP when flying with other GA aircraft. I can only reduce power to 0 MAP, so the other pilots and especially the Lead must know that they cannot decelerate suddenly without causing me to overshoot. I and everyone else flying formation should know how to overshoot safely.
The Legacy with a large C/S prop has much better air braking than my LNC2, so should be a bit easier to fly and decelerate alongside dirtier aircraft.
The minimum safe airspeed in formation is whatever speed you need to generate the maneuverability and Gs that you need to hold position. That could be below "stall" speed over the top of a loop, or it could be over 160 kias in a 4G turn. I fly with an AoA indicator and audio annunciator, and would recommend the same. It is pretty easy for a Yak to pull more Gs that I would be comfortable with at 80-100 kias. Lead should know that I will exit the formation if they pull enough G to approach my stall speed. It's nice to have Betty let me know since my eyes are definitely outside during a formation approach.
BTW, I first met Dennis Johnson at a formation clinic sponsored for RVs. You meet the nicest people flying formation! Since we brought the sleeker aircraft, we flew together. I've flown with many other Lancair pilots here on the LML, but I will let them unmask themselves.
-bob