You cannot mix shielded and unshielded components of a cable system. If shielded cabling is used, it must be shielded from end-to-end, and grounded, at least on both ends.
There are documents that explain things for you. For example, Shielded and unshielded twisted-pair cable revisited:
If STP cable is combined with improperly shielded connectors,
connecting hardware or outlets, or if the foil shield itself is
damaged, overall signal quality will be degraded. This, in turn, can
result in degraded emission and immunity performance. Therefore, for a
shielded cabling system to totally reduce interference, every
component within that system must be fully and seamlessly shielded, as
well as properly installed and maintained.
An STP cabling system also requires good grounding and earthing
practices because of the presence of the shield. An improperly
grounded system can be a primary source of emissions and interference.
Whether this ground is at one end or both ends of the cable run
depends on the frequency at which a given application is running. For
high-frequency signals, an STP cabling system must be grounded, at
minimum, at both ends of the cable run, and it must be continuous. A
shield grounded at only one end is not effective against
magnetic-field interference.
Shielding on a cable primarily makes up for poorly designed/constructed cable. If you have both shielded and unshielded cable with the same ratings, you really gain no advantage using shielded cable. Properly installed, both will have the same characteristics of noise immunity, and improperly installed shielded cable will be much worse.
Your power cables run at a vastly different frequency than network signals, so you should not expect interference from that. A bigger concern is the applicable law. For example, in the U.S., it is not allowed to run power and network cables in the same path, and the AHJ (building inspector, fire marshal, etc.) can red-tag the building (preventing occupation and use) and levy fines until the situation is corrected.
When running cabling, you should also consult the ANSI/TIA-569-C Commercial Building Standard for Telecommunications Pathways and Spaces. Here is an overview document, and what it says about that:
Power Separation
Co-installation of telecommunications cable and power cable is
governed by applicable electrical code for safety. In addition, the
following precautions should be considered in order to reduce noise
coupling from sources such as electrical power wiring, radio frequency
(RF) sources, large motors and generators, induction heaters, and arc
welders;
- Increased physical separation
- Electrical branch circuit line, neutral, and grounding conductors should be maintained close together (e.g., twisted, sheathed, taped,
or bundled together) for minimizing inductive coupling into
telecommunications cabling
- Use of surge protectors in branch circuits can further limit the propagation of electrical surges.
Use of fully enclosed, grounded metallic raceway or grounded conduit
or use of cable installed close to a grounded metallic surface will
also limit inductive noise coupling.
The NEC (National Electric Code), NFPA 70, also has much to say about cabling. It has been adopted by all 50 states and most local jurisdictions. It doesn't just deal with power lines; there is quite a bit of it which deals with low-voltage (network) cabling.
Commercial building cabling is not a DIY project anymore. The frequencies used by modern networks, and the codes, ordinances, legal requirements, etc. of cabling all make it necessary to have cabling properly installed and tested by certified cable installers. The test equipment necessary to properly certify cable can cost several thousand dollars, and it requires experience to use properly and to be able to correct any problems discovered during testing.
