New player with a myriad of space questions

Hello all! I'm a relatively new player, having only played for a few weeks now. I've been spending a fair amount of time online doing as much research as possible, trying to keep myself from having to ask questions. So far, that has worked well, but I'm at the point where I need a few clarifications.

I guess the basics for me are; I'm a Federation Romulan currently flying a T5-U D'Deridex Retrofit. For the most part, I have been following this build: https://www.reddit.com/r/stobuilds/comments/3jjwuw/t6_fleet_dderidex_hse_tank_70k_dps/. The build is over two years old, so I honestly don't have any idea how bad off I am for following it. Also, I'm in a T5-U Retrofit and not the Fleet T6 Retrofit, so I know there are some limitations.

Now, on to the questions!

1. My ship seems TERRIBLY slow (Bajor Defense Engine at XII). From what I have read, the D'Deridex ship is slow and I understand that. However, when I do Red Alerts, such as with the Borg, others zoom past me like I'm sitting still. I'm lucky to get any shots off on a cube or sphere before the final wave arrives. Is this normal? I'm considering dropping EPtoW, KLW, and APB for APtoID, EPtoE, and APO. I'm also looking at the Impulse Capacitance Cell Console. Am I on the right track with these ideas to get a speed boost? These ideas no good? Am I beating a dead horse here?

2. Which Reputations and R&D should I be working on for my ship? Right now, I'm dedicating my R&D to Beams so I can get Beam Barrage. As far as reputation, New Romulus is almost Tier 3, and I'm almost Tier 1 with Iconian. I'm basing my choices off the previous guide I linked. Should I be focusing on more than just these two?

3. I've searched it and seen the question asked, and most responses are typically, use what you like. Regardless, I'm going to ask it anyway. It looks like a lot of the newer build's (for other ships) I'm finding are using disruptor beam arrays. I've managed to gather eight Antiproton Beam Arrays MK XIV [CritD]x2 [Pen], and that's what I have been using. Are disruptors better overall? Should I go for disruptors, or work on upgrading what I have?

4. I am not in a fleet. I get invites everyday, but I don't accept for two reasons. First, I don't know if these invites are from random people who have just made their fleet or not, and second, since I am so new I don't want to be a burden on a more established fleet. Are my concerns unfounded? If so, should I start looking into a fleet? Also, is there a good place to look for fleets? Some of the consoles and ships I want are tied to being in a fleet from what I understand, so I do ultimately want to join one.

5. Lastly, a bit of a space barbie question... The Flambard Science Dreadnought Warbird, can you change its appearance to that of the T5 Scimitar Dreadnought Warbird? If so, does it come with that appearance as an option or would I need to get the T5 Scimitar as well? EDIT: I ended up getting the Flambard, and it does have the Scimitar appearance available by default. I do not own the Scimitar either.

I think that is everything. I appreciate anyone who takes the time to answer any of these questions! I'm at work, so I don't have access to my ship to post my current build.