Is it permissible to say "May Allah reward you with a thousand good deeds" or "May Allah reward you on our behalf and on behalf of the Muslims with all good" to one who does good, and is there in that a limitation of Allah's reward?

There is no objection to saying "May Allah reward you with a thousand good deeds" and similar phrases, as the intention behind them is to exaggerate in supplication. Nevertheless, it is preferable to limit oneself to "May Allah reward you with good."