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When you set up your security group, it will be Active/Standby across the two sites already, you don't have to adjust anything for that. SGMs in site 1 will process traffic, SGMs in site 2 will be in standby in case a failover event occurs. 

You can mix & match SGM appliances in a running security group, but we generally wouldn't recommended it for a first time out. It can get complicated and you lose some of the quality of life features (such as auto-cloning). Limitations apply as to which models you can run together. See here for details: https://support.checkpoint.com/results/sk/sk162373

If you are mixing appliances, it is recommended to have the same mix across sites, yes. Further to that though, we do recommend that you have the same SGM setup on both sites, so that you maintain full high availability. If you need 4 SGMs on site 1 to serve your network load, it stands to reason that you'll need the same capability on site 2 so that you have continuity if a failover should occur. Hence there should also be an SGM 3 and 4 on site 2 as well.