Can you clarify what you mean by paying "equity and commission"? It is not unusual to pay someone just commission for seeking beta customers. If you are paying a reasonable commission, then a small amount of equity as a "retainer" makes sense, but you don't need too much equity if there is a decent commission.
To answer your questions.
1. One reasonable approach to deciding how much equity is to base it on what a sales person would get if they were a full time employee, and pro rate it for the work/time. E.g., if they would get 0.5% vesting over four years under a normal plan, for full time employment, that translates to about .01%/mo. If they are going to spend three months half time seeking beta customers, you just do the math to get a reasonable idea.
2. Vesting over a period of years is for long term employees, to encourage them to stay. In this case, if you are hiring these people as permanent, long-term employees, you could do that. On the other hand, if they are temporary employees or consultants, at least initially, then just do 5-10 year warrants.
3. Investors will insist on a stock option plan that sets aside a specific pool for employees/consultants, and an option "budget" that outlines about how much each type of employee would receive.
4. Clarify the circumstances under which the person earns both the equity and a commission. E.g., do they get a commission for a qualified lead who participates in a demo, or does the customer have to close. If the latter, does the sales person have control over whether the sale closes?
5. With a beta customer for an untested service that has no current market or brand, it would often be a fairly high %, perhaps 33%. Recognize that it is often harder to find beta customers for an unknown service, than it is to find customers for an established service/product.