Air carrier First Officer reported a traffic conflict with another aircraft while in IMC weather on a ILS approach. The flight received a TCAS RA and followed the guidance then landed safely.

Narrative

Day 0 around XA:05; as First Officer (FO) on flight ABC in Aircraft X; we were being vectored for the 18R ILS approach in DFW. The Captain was hand flying the aircraft and I was managing the panel and the radios. About 19 miles on a base leg; approach assigned a heading of 090 and 190 kts; and a descent from 4;000 to 3;000 feet. In the turn to 090; Approach notified us of traffic at 1 o'clock and 2 miles; west bound; Aircraft Y at 2;500 ft. I replied 'looking' and then we entered IMC at the bottom of a scattered layer. Shortly after being established on heading 090; Approach instructed a turn to 150 degrees and to intercept the localizer for 18R. The Captain called for a speed reduction to 180 kts. The flight was turning from the 150 heading and joining the localizer course. Leveling at 3;000; Approach Control called and said to maintain 3;000 and cleared for the ILS 18R approach. At the same time we received a TCAS alerted 'traffic; traffic;' with an amber target -04 (400 feet below our aircraft). A second later we received a TCAS RA; the symbol turned red with a -03 (300 feet below our aircraft) and TCAS announced 'monitor vertical speed;'. The Captain initiated a slight climb about 300 - 500 FPM as we began to join the localizer; climbing to about 3;100 MSL. I made a radio call to ATC alerting them to the close proximity; within 300 feet of the GA aircraft and that we were IMC and not clear of the traffic. After I made that call another pilot radioed 'blocked'. Then TCAS alerted 'clear of conflict' and the TCAS display showed the traffic behind us; and we continued the approach.I then made a radio call and told ATC that we were clear of the traffic. A short time later the Captain made a radio call to Approach to make sure they understood that we had been very close to the GA traffic; within 300 feet; and it was uncomfortable. The Captain was unsure if ATC heard my radio call about the close traffic due to the 'blocked' radio call. Neither the Captain nor I used the term RA (resolution alert) when reporting the traffic due to startle effect and confusion as to whether a monitor vertical speed" is a TA or RA.We continued the approach and landed safely. Cause: With GA aircraft flying below the ILS 18R glide path in the bottom of a scattered layer; and so close to our altitude; maybe we should have been held at 4;000 ft longer to maintain separation."