Once you get the merchant account you can run whatever you want as long as its in the pricing of products you submit to them. You say high risk so I assume its adult or over the phone biz op (most popular high risk) The high risk factor will be the likely amount of chargebacks you will receive. The person setting up your account will let you know if you fit in this category and they will most likely give you a high merchant reserve from 10 - 30% with holding from your gross transactions that they keep in case of these chargebacks. The secret is to prevent chargebacks is with good fulfillment. If its in your name you will want to gain trust with the merchant company so they can get familiar with your transactions then open other merchant accounts with your good merchant history, either with the same company or open others. What the big accounts (100k + a month) you will need a website with a simple price sheet of what you are charging and what products they are getting. If non tangible it will most likely have a reserve. Most big companies will give you the choice of what gateway to choose. Authorize is good and chargeback gardian is what I would recommend.
This is actually my first day on blackhatworld, but if any other questions if you can pm me I would be glad to help. Good Luck