Hey Peaches, I'll try to answer best I can, but other more successful trainers are probably better to listen to
What's a good stable size to start? Should I upload new horses at the mid season upload window?
Depends on how good you feel your team is, I have rarely ever added mid season and try to prepare my team for the start. If it was me, I try and fill up to maximum, regardless of how good they are, throw enough s**t at a wall and some sticks ha.
Does race location matter? (I'm U.S. based)
I don't think it does, but I'm not sure, I think you are fine adding US schedule horses as long as it's a league mode game
Do I enter my horses for races all at once or week to week? Are there rules on how often I can enter a horse or how many horses I can enter? (I think I remember seeing two max)
Yes two horses max per race, depends entirely on conditions, some races may be 1 max from memory. I always try and do it week to week and once the results are in, so I'm not rushing to the deadline
Do I submit my horses with a rating? Do I have to change their rating from week to week or is that automatically done?
You submit with a rating then the AI handicapper does their thing. There should be a stickied how to guide in the forum as to how to do this in CK, but from memory you need to change all horses ratings in the CK to 75, there is a shortcut to doing this by clicking on one horse and changing it to 975, this changes all horses ratings in CK to 75. I think for NH it might be 100 but someone else can confirm (same shortcut by using 910? I think?!) If you are adding a horse who has run in the league the season prior, and is now a year older, you should enter it's handicap mark. I.e. I had Leipzig Lady as a 2yo rated 85 I think, if I upload her 3yo version for this season, I need to change her rating (to the 3yo version) to 85 in the CK before I export from CK
When do I start using my own stallions instead of gamebreds for breeding? (I feel like I can't use my own until they get to 100% potential)
I tend not to do this until I get some very good horses 85-90+ potential, that are winning 5+ G1 races, but I'm not the most successful so this isn't probably the best starting point.
Why am I getting the impression from the community that the game races and CK races behave differently? Shouldn't there not be a difference? (I am pro racetrack breeding theory but understand that I have to trial at the CK instead of relying on race results)
Yes this is the case, unfortunately, I've found a 6f horse this year in game, who's rated 95 and can't buy a win, yet is very competitive in CK. Howwwwever, I do tend to find, this is very rare, and most of my league horses have been G1 winners in game.