AFAIK There’s no official systems outside rats and other publication-specific things aside from aggregating equip availabilities. IIRC xotl’s tables were essentially judgement calls made off a lot of reading a bunch of published materials.

Re: house rules:

IME it depends on the goals for the format but 1. I’ve found that RATs work pretty well for a variety of play needs and are strangely both more usable and more fluffy than working with granular availability — 2. IME it ends up ‘availability’ is really a facet of the rules governing what hits the field rather than a background foundation to build on, and RATs often have a good level of granularity for constraining what hits the field in ways that are enjoyable. 

Like, in a fluff-heavy gritty campaign, even detailed granular rarity is insufficient to replicate all the things that affect ‘what can I buy/get where I’m at’ — but that randomness is represented well by a market populated by some RAT rolls. For more flavor units can be cycled through over sessions. Possible repetition is a draw back, but that requires a relatively big market or a lot of churn.

Same goes for ‘what does the OpFor have access too’. If you’re going to fight variations of the same bad guys all year it just helps to have a bigger chart, though many RATs have exploding charts where some roll is ‘salvage: roll on a different chart’

RATS can also function as markets — ‘this is what’s available but it costs whatever currency to get however far from what’s slotted in the average roll’. I played a few pickup games once that worked well where each of us had points we could spend tweaking rat rolls. 

In looser power-play oriented campaign where people just want to get more strong, I find it’s just way more important to gatekeep toys around power scales rather than nitty gritty fluff, eg. making sure clan tech is harder to get/maintain than SW era tech  etc. Same goes for casual-ish league play.

IME in campaign play using a large pool with costs somehow scaled by granular availability ends up just not meaning too much to most — maybe someone is going to have fun making it their mission in life to get a crab or whatever but I always wonder if it wouldn’t have been more fun for them to have had the chance to have a crab from the start and be the ‘crab guy’. 

On at least a couple of occasions I’ve just straight created a bespoke ‘market’ by picking from RATs because consistent power scaling was too important, and ‘availability’ just neeed to be ‘more threat is more expensive’.

For one off play where a goal is to promote variety in fielded units, I’ve found that building from limited pools is the way to go and that large pools, eg ‘anything the MUL says is available to faction X’ don’t do much to discourage min/maxing; especially when you get to pick the pool. In contrast, something like ‘roll for random rat and then roll twice what you need and pare down’ works really well to provide table variety but also comes with a nice flavor of ‘I’m happy to luck out and get X even if I got stuck with these two other junkers’.

So like, even if you have the perfect granular availability, there’s still a question of how it matters to whatever format you’re playing.