The "requirement" for 750 SIC hours can be waived by the company, and they consistently have for a long time now. Just to clarify/reiterate what others have said, here's how a you become a "street captain" at Horizon. Currently, this is only possible for the Q400 (junior E175 CA award was 8/2017 last bid).
1. Get hired (as an FO).
2. Put in your bid for a Captain vacancy. Captain vacancy bids open on the 1st of every month and are awarded on the 22nd of the month. For example, the Dec 1st bid is for February upgrade classes.
3. If you have less than 750 hours at the company, they will ask you to complete an upgrade application and provide a resume. The upgrade review committee will either approve or deny your upgrade, based on your experience. There are many people in the last two years who have been approved with very little or no company time. Some people decide to bid just after finishing FO IOE, and others have been awarded CA without even getting to initial FO sim training. The last bid had a Q400 CA awarded to a 5/2019 hire (obviously way less than 750 hours).
4. After getting the award, you'll have to wait for your CA class date, then finish CA training. Best case scenario, you won't finish CA OE until about 5 months after your DOH.
It's hard to say exactly what kind of experience it takes to get approved to be a Captain with zero company time, and I believe that they have gotten a little more picky because of some challenges with street captains in the past. The upgrade application will ask about your total time, 121 experience, recent flight time, CAT II/III experience, winter experience, etc. If you think you are strong in these areas, then you probably can get approved immediately. Otherwise, if you do a good job in initial FO training and OE and meet 121 requirements, you can probably get awarded the first bid after OE.
As far as Q400 CA vacancies go, they haven't filled all of the posted bid vacancies for over a year. They usually post 12 vacancies and only award 7-8 each month. So it's really not a question of seniority to hold Q400 CA, and it hasn't been for a while.
The catch, as others have noted, is airport reserve. The junior 2-4 lines at any base are dedicated airport reserve lines, meaning you have 15 or so airport reserve days scheduled for the month. Airport reserve lines are either AM (5ish to 1ish) or PM (1ish to 9ish).
Airport reserve is miserable enough, but the worst part is that the company will use airport reserve pilots as an absolute last resort. Even if there aren't any other reserves available, they'll offer the trip as premium pay open time before giving it to an airport reserve. It's a frustrating situation that makes it very difficult for junior pilots in either seat to get any flying time. The only sliver of hope is that staffing is so short right now that even the airport reserves are getting used eventually (although not for a lot of block time). Overnights for airport reserves are rare, so plan on a lot of time in base.
And of course, it's hard to get off airport reserve, because people are always upgrading in front of you as they become eligible. It varies a little by base, but basically you can't expect to get off airport reserve until you have the seniority that captain would normally have (approximately 2 years). There are definitely street captains who are still on airport reserve after a year and a half.
All that said, it is still possible to get a very quick upgrade in the Q400, and it might be worth it for you depending on your situation. If you desperately need 121 PIC and don't have the patience to wait for it elsewhere, this is a way to get it.