I don't think there is a soviet player guide for TFH. That said, I've been playing as the Soviet Union for ages, so I'll try to answer your questions. It should be noted that I haven't always taken the most 'efficient' route for role-play reasons. (see AAR in my signature)
500 IC? Is that base IC, or effective IC? If it is the latter, with what laws and tech upgrades?
In my current game, I balanced army expansion and practicals with IC construction. Poland was conquered on the 4th of October 1939. At that time, I had 226 base IC for 299 effective IC with 'Full Mobilisation' and up to date industrial production tech. This is, of course, less than it could have been. I also had 641 brigades, 25 air wings, and 39 naval units.
In June 1941, my target for army expansion I had 240 base IC for 324 effective IC with 'War Economy'. I also had 958 brigades, 51 air wings, and 45 naval units.
It should be noted that I didn't take Bessarabia by event, but annexed all of Finland.
To my surprise, Germany only invaded in June 1942, at which point I had 240 base IC for 324 effective IC with 'War Economy' -> 438 effective IC with 'Total Economic Mobilisation'. Also 1,068 brigades, 65 air wings, and 49 naval units. This is proving to be sufficient to hold back German forces on normal difficulty with my armies controlled by AI (I give each army objectives and a stance Defensive/Agressive/Blitzing) and my Air force and navy controlled manually. I also built a lot of infrastructure and a significant number of land forts.
Now, the amount of IC you build is constrained by three factors: Resource availability, starting Industrial Capacity, when you need to have the largest possible army.
1. Resources. You can go several different ways here. The Soviet Union has lots of resources, 738 Energy/day, 280 Metal/day, and 156 Rares/day. With each unit of IC using 2 Energy, 1 Metal, and 0,5 Rares per day, the limiting factor here is clearly Metal. With the 'Full Mobilisation' law, you can thus support 280 effective IC, with 'War Economy' that becomes 10% more or 308 effective IC, which corresponds to 246 base IC. With 'full economic mobilisation', you can support 350 effective IC, which corresponds to 233 base IC. So, if you want to be self-sufficient with no need to trade or stockpile, your ceiling is 233 base IC with tech modifiers. I pretty much aimed for between 233 and 280 base IC as a relatively realistic expansion.
Of course, you're going to be trading and stockpiling, so then you should probably do some calculations to make sure you can sustain your industrial base all the way to 1945. With a full Metal stockpile and no wartime trading, you can support 91 IC (for 3 years) more than the amount you can support with your metal income. Any amount of metal you can get consistently by trading can be added to the total amount of effective IC that is sustainable. (until you reach the limits of rare materials production, of course)
2. The second factor is not really limiting for the USSR as you can pretty easily build over 600 IC by 1939 if you prioritise it over anything else. Of course, you have to consider whether you want to keep up with some important practicals or whether you're going for an all out expansion.
3. Here the question is when to stop building IC and to start building units. This is most relevant in the case where you're trading like your life depends on it and maximising IC construction. When you're building IC, you're not building units to fight your war. You can calculate how long it will take for your IC to pay for itself starting from the start of construction of the unit of IC:
The formula is: (IC production time x (IC production cost + 1) ) / (Effective IC / Base IC (not counting Lend-Lease)) = time for IC to finish paying for itself from start of production.
This should give you an indication of when you should transition to building your army depending on when you want to have your army ready.
Additional factors:
Practical values, which mean that if you go for an all out IC strategy, you should start a partial transition to warm up practical knowledge before the transition point you would calculate using the formula above.
Reinforcement and mobilisation: count an additional month or so to reinforce and mobilise before your army will be ready.
Economic law improvements (and to a lesser extent Industrial Production tech upgrades) make your IC pay for itself faster, so if you expect to be changing your economic law soon, you might want to incorporate this change in your calculations.
For example, you want your IC to have paid for itself by June 1941: New IC costs 5 IC and takes 12 months to produce (this would be a lot lower if you've been building IC for a while) you have 'War Economy' as your economic policy, and you get a 15% boost from Industrial production technology. (but Stalin takes away 5%). (12 months x 5+1) / (1+0,25+0,15-0,5) = 72 / 1,35 = 53,3 months. In this theoretical and totally unrealistic example you would build your last batch of IC 53 months and 10 days (or 1600 days) before the 22nd of June 1941 or on the 3rd of February 1937. This is a lot easier to do if you just have the formula in a spreadsheet, of course, as you would have to recalculate before building each new IC when you get close to your initial estimate as costs & laws etc will have changed.
Hope this helps.
