I have converted a couple of hard tale Cannondale which have conventional rectangular frames and built an eBike using a 29er carbon frame with carbon seat post, integrated bar, titanium BB.
I prefer using rear-wheel mounted electric motor conversion kits because they have more power/weight and better drive characteristics if you balance the rear motor with a triangle battery pack. This is the arrangement the Israeli military and other special units forces have come to use. They build custom center triangle compartments that attach via locked brackets for 30 second swap-outs. The triangle battery can be large. Using a large battery means the weight has to be distributed lower and more central.
If range and power are not a priority, then you should determine what battery you will use and then measure the frame... measure it again. See what others things as you are doing here. The first choices are how you will use the bike... how far, hot fat and lazy you are (being funny), or if this is one of multiple eBikes you plan to play around with so can build multiple ones to suit.
Suspension on eBikes is not important to me: a $120-$200 suspension seat post with good air shocks in front can do all you need. eBikes are heavy and too much suspension can lead of the '1950s Cadillac ride' phenomena. That can be bad on a two-wheel vehicle which is why motorcycles have much stouter frames and suspensions.