I hope I'm not too late to help with this. The difference between the Family class and spouse in Canada class is the requirements. If you choose to apply as family class, your application will be considered "outland" and you will not be able to apply for an open work permit, and legally cannot work. You WILL however be free to travel outside of Canada without your application being in danger of being revoked. You may still live with your husband on either family class or spousal in Canada.
If you apply under the spouse in Canada class, you MUST be physically present in Canada for the duration of your application. If you are found to not be in Canada at any point during the application (i.e. you leave Canada and are not allowed back into Canada when trying to return) you application will be tossed and you will not be able to appeal. Under this class, you are able to apply for an open work permit which you will submit along with your PR application. The open work permit is not a permanent program, so from what I'm reading it may not exist come december should it not be renewed. If you plan to apply inland, I would recommend looking more into this.
As far as proof goes, I would recommend starting to accumulate proof now. If you are going to be living in Canada, you should have your husband add a cellphone to his plan (if he has one) under your name so that you will receive statements to the address you will be residing at. If you husband receives supplemental insurance through his work, tell him to speak with HR about getting you listed on his insurance benefits. If your husband has a credit card, tell him to add a supplemental card in your name so that your name starts showing on statements. If possible, open a joint bank account as soon as possible. If you husband is renting, see if he can get you added to the lease. If you are ultimately unable to provide ANY kind of proof that you are living together and dont want to wait to accumulate some, you have two options: you can either 1) write a letter stating all of your circumstances on why you cannot or do not have the appropriate amount/type of proof required, and what you will be doing while the application is in process to start creating a conjugal relationship (meaning combining finances, and creating a more married-like relationship) now that you are reunited with your husband.
OR
2)You can apply as outland and live in Canada with your husband while the application is being processed. Since this class doesn't require proof of combined finances and lives, you wouldn't need to provide things like join bank account proof or a lease with both of your names on it. Instead you'll need to provide proof that your relationship is genuine, which, with a kid that your husband is the father of, will not be hard at all.