Blending powders is a risky business if you don't have specific knowledge of the process and equipment to do the proper ballistics/pressure testing. Ammunition mfg. both commercial as well as Gov. arsenals blend powders to achieve a specific burn rate.
There are also some loads that use what is called a Kicker or Duplux load which is a small charge of fast burning easily ignited powder on the bottom of the load column topped by a heavy compressed charge of very slow burning powder which keeps the powders from mixing together,it want hurt if it does mix somewhat but velocity variations and poor accuracy will result.
The very slow powders used in the duplux load would only be used in a situation where the powder by itself is way to slow for the application an want produce enough pressure on it's own to be very useful without adding the kicker. I personally don't do it or have a use to as I have none of those very slow powders or type cartridges that would benefit from the use of those type loads but there is a wealth of knowledge past and present about there use.
To the OP's original question about only having 6 powders to cover all you needs,that really depends on the individual an what they want to accomplish with a particular caliber. Based on type of shooting,ranges I shoot at and what the particular caliber is and used for as in target shooting,hunting or SD I can easily stock 6 powders if I want to have choices. I can choose fewer powders if I just want a powder that will work in multiple calibers that give me a fairly decent latitude on load ranges and will work very well,it may not be the best choice but it does what I need it to do.
I have to point out that although I do shoot an occasional jacketed bullet about 99% of all my pistol and rifle shooting is done with my own cast lead bullet which I cast from different alloys and different nose styles for a given purpose,velocity and pressure and after doing so for the past 30 years I've never felt limited for the type shooting I do. Velocity isn't really an issue for me as I can pretty much match any handgun jacketed load I shoot with cast lead and in rifle I'm pretty much limited by the twist rate of the bore and the bullets weight / length when it comes to usable accuracy. I shoot 4 different caliber handguns and 4 different rifle cartridge in both US and Russian 30 cal.
In my handgun calibers I like Bullseye for all the non magnum loads,for the 357 Mag. 2400 is my go to powder. Unique will work fine in all these calibers as well.
9MM
9 x 18 MAK
38/357
45 ACP
In my rifle cartridges I mainly shoot loads composed of 2400 but I also have reloader #7 & H4895 with will work with both jacked and cast loads. The H4895 can be used with the 60% starting load with cast lead and be worked up to a suitable velocity that a particular bullet /twist rate will allow which with the rifles like my Mosin and SKS with a 1:10 are around 2K fps. and the 1:12 twist bores are a couple of hundred fps. faster. Bullseye also work for me with light weight cast pistol bullet for sub sonic plinker/small game rifle loads. I shoot the same 32 cal. Lee 90 gr. tumble lubed SWC sized to either .311 or .314" in all my rifles.
30-30
300 Sav.
7.62 x 39
7.62 x 54R
So my powder choices are
Bullseye
Unique
2400
Reloader#7
H4895
And Reloader#15 to round out my top 6.