If I'm reading this right(and looking at the final picture), the beam itself is sagging. It may have reached an equilibrium point, and simply jacking up an reattaching it will fix it. But you'll still have a bowed beam.
My inclination is, a properly built beam should be able to span that distance and hold the weight without sagging. I have smaller, longer glulams spanning my basement, supporting two stories. But maybe someone who knows a bit more about structural engineering will chime in.
If you want to create one yourself, I would use like 3 2x12's laminated vertically. Boards are much stronger vertically oriented than horizontally. Think floor joists or door/window headers.
All loads on beams / columns matter. So, if you are adding beams (loads) to existing beams, you are right to worry about its affect. However, there are three things in your favor: 1) everything is designed with a "safety factor", 2) residential design is way-over designed, and 3) beams are designed to fail in tension.
1) Structural design has "Working Stress" and "Ultimate Stress". Ultimate stress (failure) is about 150-180% more than working stress (what we design beams, columns, etc. to).
2) Residential design is largely done by non-professionals (architects and structural engineers). So, "what is commonly used" in residential construction is grossly over sized.
3) Beams are designed (design formulas used) to fail in tension. That way, we'll see cracks and presumably investigate, rather than fail by compression, which will fail with a bang...and have a disaster.
So, back to your question, yes, there is a way to determine how much weight can be added to a beam, how big holes (and how many holes) can safely be drilled into a beam, etc. However, unless you're "doubling the load" or excessively drilling an existing beam (1 connector is not excessive), then I think your safe.
You can destroy a concrete beam quicker than steel or wood beams. When drilling the concrete beam, you'll know quickly if it's a poured-in-place beam or a steel beam encased in concrete. In fact, you may not be able to drill through the steel/concrete beam. So, rather than "mess with the concrete beam", why not design a "saddle" to fit over the beam and support your new added beam and lights.
If you decide to proceed with drilling the concrete beam, I'd recommend you contact an architect or structural engineer (not a CIVIL engineer.)
Best Answer
TL;DR - they're both designed for construction purposes and meet standards*
(*in the US, at least)
Generally, Simpson StrongTies are used for all sorts of construction purposes. Read through their documentation to ensure this will meet your needs. Since you don't indicate what you're building, it's really hard for us to say for sure.
The ATP4 is for 4-by material (~100mm) while the ATP6 is for 6-by material (~150mm), so make sure you're getting the correct sizes for your project. Also, be sure to use the proper screws (they specify that it has to be their structural screws and washers, but I'm sure any similarly spec'd structural screw & washer will do the trick - their's will be powder coated to match, which is an advantage) and to fill all the mounting holes in the bracket.
Personally, I'd be a bit concerned by the fact that the ATP4 has only one hole for each piece of wood. In my mind, this makes the cut of each joint extremely important. You'll need to leave some room for expansion (no more than 1/8" ~3mm gap), but too much leaves you with a pivot point on that single screw. I doubt that this would fail any certification process they put them through, but it would make me a bit nervous because I know my ability to get consistent quality joints.
The MiTek is also used for many construction projects. Personally, I'd feel a bit better about this particular strap as it's got 4 nails going into each piece of wood that it's attaching to, so there won't be any pivot issues.
Of course, the MiTek is "ugly" looking galvanized steel while the Simpson is "pretty" looking powder coated (not sure if it's also galvanized - I didn't look for that, but I'd be surprised if it wasn't) steel. On that end you're comparing aesthetics vs price and that's a decision only you can make.
I've used both Simpson & MiTek on projects at my house (a garage, deck and storage shed), but I've never used the decorative ones, as mine have all been buried and not for show. I've been happy with both and my choice was made mainly on what was available on the day I purchased.