I don't have rebellion (yet ), and I haven't played against human players (I've started to play against Hard AI's), so I can't give you advice for those. However, I can tell you what I'm currently using.
I prefer to use a single massive fleet with a mix of everything really. When I have large fleet supply and command crews, that includes an Akkan, a Sova, a Marza, a Dunov, 2 Kols, greater ratio of Javelis to Cobalt and Cobalt to Garda (around 30 Javelis, 24 Cobalt and 18 Garda), around 6 Percherons, 8-12 Hoshikos (not that they end up repairing as much as I'd like ), 6-8 Ogrovs, 3-4 Cielos and 10-15 Kodiaks. This means I can walk into most enemy fleets or planets and win with at most moderate losses. The large numbers are somewhat a reflection of the approximate ship numbers in smaller fleets, with a large number of Javelis and Cobalts, some Gardas, then introducing cruisers in this order: Hoshikos, Percherons, Ogrovs, Kodiaks and Cielos.
I also have a fairly weak, amateur fortification strategy focused on border worlds; I usually only have at most 1 repair platform and 2 hangars at any other worlds. Border worlds will have about 2 repair platforms, 3 hangar defenses and 6 Gauss platforms, all close together and patterned for maximum coverage. I also usually dump a star base at the border worlds with a healthy mix of offensive modules (typically 2 in weapons, 2 in armour, 2 in hangars and 2 in safety override) for added security, leaving my fleet able to pursue offensive actions instead of being held back on assault response and relying on static defenses to hold off enemy fleets (unless I know that the defenses will be overwhelmed). I have a feeling that I'm going to have to mix this up so that I can adapt to the harder AI's.
As for fleet positioning, I keep my über-fleet at my border worlds at all times for immediate assault response and spearheads. The only time I take my fleet away from the border is if I think I'm going to lose the battle, the world I'm defending, or taking it home on some sort of victory march.