Greetings CurtLeeN,
First, I am not aware of anything would prohibit you from using Type SE-R in your configuration to the wirewayitself as long as you meet all the rules associated with Article 338. You do have to keep in mind that Type SE Cable does have some ampacity limitations and so on that are found in 338.10(B) to be aware of so I suggest you review them well prior to undertaking this task. However, as infinity states, the issue of the tap conductors would not permit Type SE from being used based on 240.21(B)(1)(3) so at that point you would have to transition.
In regards to the basics of proximity you describe I would assume (assuming always gets me in trouble) that the 10 tap rule under 240.21(B)
240.21(B)(1) Taps Not over 3 m (10 ft) Long. If the length of the tap conductors does not exceed 3 m (10 ft) and the tap conductors comply with all of the following:
(1) The ampacity of the tap conductors is
a. Not less than the combined calculated loads on the circuits supplied by the tap conductors, and
b. Not less than the rating of the equipment containing an overcurrent device(s) supplied by the tap conductors or not less than the rating of the overcurrent protective device at the termination of the tap conductors.
Exception to b: Where listed equipment, such as a surge protective device(s) [SPD(s)], is provided with specific instructions on minimum conductor sizing, the ampacity of the tap conductors supplying that equipment shall be permitted to be determined based on the manufacturer’s instructions.
(2) The tap conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply.
(3) Except at the point of connection to the feeder, the tap conductors are enclosed in a raceway, which extends from the tap to the enclosure of an enclosed switchboard, switchgear, a panelboard, or control devices, or to the back of an open switchboard.
(4) For field installations, if the tap conductors leave the enclosure or vault in which the tap is made, the ampacity of the tap conductors is not less than one-tenth of the
rating of the overcurrent device protecting the feeder conductors."
Now, consider all those items that are in "BOLD" and "UNDERLINED" in your quest. Once you establish all that then look at 440.22(B)(1) and the math should become very clear at that point based on the information you actually know about each unit.