Pam's is a wonderful master clock and is one of the most common modules in all of Eurorack but if you're using your MPC live for your clock then there's really no need for it in your case.
I agree with noodle_hut about getting an active mult instead of the passive. I prefer to use splitters or stack cables over passive mults because they take up no rack space. If you're going to spend rack space on a mult, make it active.
If you need VCAs there are tons of options. Intellijel Quad VCA is very popular, it's capable and it's reasonably priced. Mutable Instruments Veils is also very popular; MI is out of business now but there are many clones available. Other good basic VCA choices are Xaoc Devices Tallin and Doepfer A-130-2. Tallin can be used as a standard VCA but also has some pretty sweet overdrive/distortion if you want that. Another good VCA option is the ADDAC802 or 802R. These two modules are functionally the same, the only difference is the -R version re-arranges the location of the knobs & sockets. This can be used as 5 individual VCA's, as a 5-channel mixer, or various combinations of mixing and VCA use. The cool thing about this module is that it has a three-position switch for every channel, so you can instantly mute a channel by switching it off, or you can choose which output you send the signal to, so it can act like 2 mixers at once.
"Mixers" is a broad subject in Eurorack and can be nearly anything. Some are tiny and simply add signals together with no real control, say the Journaloge Add2. Then on the opposite end of the spectrum you have something like the ADDAC807 which is huge, very expensive, and is basically the same thing as a full featured studio mixer. It's hard to give advice without knowning what specifically you want to use the mixer for and there are so many options, but for small systems I like the Modulaire Maritime V2AXP. It's compact and has CV panning so you can also use it for cross-fading your sounds into your stereo mix. However, I don't know how useful the mixer would be for you because currently you only have one sound source: Ballista Blast. I'm assuming you would route that through Multigrain and then to your output. There's no real need for a mixer unles you add other sound sources (oscillators or voices) to your rack.
Likewise there are zillion options for envelopes. Xaoc Zadar is a very popular quad envelope, but it's also not that 'playable', if that makes sense, because of its controls. Many people prefer more old-school ADSRs with a knob or slider for each control, but those often take up more room in your rack. However I am not sure that you necissarily need them. Envelopes are important for basic subtractive synthesis: That works by having an oscillator constantly producing a signal and the envelope is what "lets the sound out" to play only the specific notes you want rather than a constant steady tone. Usually another envelope is used to control filter at the same time. However, that works in conjunction with a basic oscillator & a seperate filter and you didn't mention any of those. Ballista Blast is a complex oscillator which already has its own envelopes and filters built into the module. Unless I am missing something I don't think you have much of a need for envelopes with what you've described so far.
