Because I am new, I don't know what environment I will work best in (small or large), but given my regular office job preferences, I feel I might enjoy working with a smaller privately owned brokerage (boutique brokerage).

Think boutique hotel. 4.

So far, all the big-box shops (and a few boutique firms) have been a hard pass on my pitch.I'm primarily interested in identifying and wholesaling deals to investors, and not terribly interested in actively representing buyers and sellers. 

46.Do you have boutique-style, single point portfolio based management services?

I would go with a smaller boutique brokerage with a broker that wants your business.  

Look for little boutique brokerages. 

So, I would look for 100% shops or boutique brokerages that may charge a low flat fee to hang your license there.You will need to have E&O insurance (sometimes the Broker provides) for deals you do outside of your own.It will also cost you to join the MLS and obtain a lock box key for entry into homes.You just need to determine whether the expense of obtaining and maintaining is worth the savings you will receive from commissions on your deals. 

I joined a boutique RE agency because at the interview they told me they train and give over leads to help me get started.

I've read several articles of national vs boutique brokers and I'm leaning towards starting off with a national brand.

Consider brokerages that cater to training newbies and providing leads, as opposed to small boutique brokerages that may give you a better commission split.