#1
I plan on Buying another Cabinet, to run with my current one in parallel, any suggestions?

I am not sure if I should just buy the exact same cabinet loaded the same, or a different one my current is a peavey 5150 slant cab loaded with vintage 30s
was thinking of getting a straight mesa 4x12 cabinet, I have heard it is best to have the same size speakers in both cabinets. If its not a big deal a 2x15 mesa bass cabinet might be nice.
#2
I'm not sure about the whole same speaker size deal - I've never heard anything about that. I know bass player's mix-match their speaker sizes (normally one 4x10 cab and a 1x15 cab).

As far as to which cab you should get, I would suggest a Mesa straight cab or a Randall straight cab - depending on whether or not mix-matching speaker size is a problem, then getting a 4x12 or a 2x15 is totally up to you.
#4
my other guitarist has a custom madison (we are endorsed my madison) 4x12 and a straight custom madison with a 15 and either 2 10's or 2 12's.
its anaweseome setup.
Fender MIA Standard Strat
ESP LTD Viper 407
Epiphone SG
Starcaster by Fender(modded with Bill Lawrence l-500-xl)
PEAVEY 6505+ (FJA transistor modd)
Peavey 65 watt transtube studio-112
Marshall 1960AV
Vader 2x12
ISP Decimator
MXR 10 band EQ
#7
15s honestly dont add that much low end...if they did you'd see many more guitarists using them.

my buddy has a randall 2x12 1x15 and the low end from a g12-75 marshall 1960a destroys it
Baron K2 SE 120
MILLS 4x12 Afterburner
Eventide TimeFactor Delays
ISP Pro Rack G Noise Suppressor
BKP Warpig pickups