If you're married to your current design then it's best to feed the fuge with a separate line which you can control. You don't want or need large amounts of flow through a fuge. You certainly don't want total system flow going through it. Many problem algaes tend to grow in the areas of the system with the lowest flow, and it's preferable that that area be out of sight. You could use a line right off the drain, either dedicated or teed off. In any case a valve is in order. Or you can use a powerhead to move water from the sump to the fuge, which might or might not require a valve. Connect the two tanks by bulkheading them both on the end taht faces the other, a couple inches above the bottom of the tanks. Position the holes so that one is toward the front and one is towards the back. Putting them directly opposite requires fairly exact placement to get the plumbing in. Put a 90 on each and connect the two tanks either with pvc or tubing. 3/4" bulkheads are plenty big unless the main tank is in the 200 gal range. 1" would be called for then and wouldn't hurt on a smaller system.The optimal design would be to have the fuge mouted above the main tank, and to have it overflow into there. The feed water can come from the main pump, or a dedicated pump drawing from either the sump or main tank. It maximizes critter survival, since a certain % will be blenderized if they have to pass through a pump on the way to the display. It's not a huge difference, but if it's feasible from a construction standpoint, it might be worth it.
