You can do that, but it will cost you more in the long run. Those personal editions would only be for 4.0. When 4.1 came out you would have to repurchase all the personals again. Just because you have access to the 4.1 with your 1 pro license would not allow those personals to use it. The licenses are different.
Your cleanest solution would be to buy a Pro license with additional installs for the primary developer. We do offer this, contact support for a price quote. It is not something you can do from the main store, it is a custom use quote.
At some point it becomes more cost effective to just go to a Corporate license though, rather than adding new installs to an existing developer.
And if these clients are work for hire that wouldn't work legally. Because work for hire requires you to give the client all the tools required to build the work they paid you for (unless excluded). That would mean it would be better to buy a license for each client that they then keep at the end of the contract. In that case maybe personal is good enough, if they don't plan to update the project frequently. You would buy a personal, install it to their dev machine and then turn it over to them at the end of the contract.
Really depends on your situation with the clients I suppose.
Jason Short