Is it proper for lower enlisted to wear full-sized medals rather than ribbons when not in a ceremonial unit or position?

PV2 (Join to see) Unless the commander has designated a uniform and configuration to be worn, you may wear full sized medals.

Mess dress is NOT worn with full sized medals. Miniature medals are worn with mess dress and under very narrow circumstances on others uniforms at the discretion Of the commander.

DA PAM 670-1 Para 20–7. Full-sized U.S. and foreign decorations and service medals
a. Where worn. All personnel may wear full-sized decorations and service medals on the Army service/dress uniform coat. Female personnel may also wear full-sized decorations and service medals on the maternity tunic.
b. How worn. Full-sized decorations and service medals, except the Medal of Honor (see para c, below), are worn in the order of precedence from the wearer’s right to left, in one or more rows, with 1/8-inch space between rows. No more than four medals are worn in any one row. Soldiers will not start a second row unless they are authorized to wear four or more medals. The determination of whether three or four medals are worn in each row is based upon the size of the coat and the position of the lapel. Full-sized decorations or medals will not overlap within a row. When more than one row of medals are worn, the second and subsequent rows are positioned so that the medal pendants on the row below are visible. The first and second rows will contain the same number of medals (three or four) before starting a third row. Second and subsequent rows will either contain the same number of medals or less than the row below. The top row of medals is centered over the row immediately below. Personnel will not wear service and training ribbons when full-sized decorations and service medals are worn. Personnel may wear U.S. and foreign unit award emblems as prescribed, when wearing full- sized medals. When full-sized medals are worn, personnel may wear up to three combat and special skill badges or metal tab replicas (or a combination) from groups 1 through 5, but may only wear one combat or special skill badge from either group 1 or group 2 one above the other, above the medals, in order of group precedence. Personnel may not wear the driver and mechanic badges with full-sized medals, and they may not wear special skill and marksmanship badges below the medals.

AR670-1 Para 20–7. Full-sized U.S. and foreign decorations and service medals
a. Where worn. All personnel may wear full-sized decorations and service medals on the service/dress uniform. b. How worn. See DA Pam 670–1.
c. Medal of Honor. See DA Pam 670–1.

Wear of the Army Service Uniform is appropriate for a Memorial Day Observance
AR670-1 Para 11–3. Occasions for wear
a. All-purpose wear. The male service/dress uniform is authorized for wear on and off duty, on and off the installation, unless restricted by the commander. The male service/dress uniform is not intended for wear as an all- purpose uniform when other uniforms are more appropriate.
b. Approved wear. The male service/dress uniform is prescribed for year-round wear for all Soldiers, unless otherwise directed by the commander. Soldiers may wear variations of the male service/dress uniform:
(1) On duty when prescribed by the local commander.
(2) At social functions of a private or official nature, either before or after retreat, and while in transit to and from such functions. The male service/dress uniform is normally considered appropriate for social or official functions off the installation, such as memorial services, funerals, weddings, inaugurals, patriotic ceremonies, and similar functions.

Regarding your other post on marksmenship badges, you don't wear them with medals as the clank against each other and you really can't see the marksmanship badges on the pocket flap anyway.

AR and DA Pam 670-1 it's ok to wear your medals at your option, if you are attending as an individual. If you are attending as a part of a color guard or other mission it's up to the commander if they prescribe medals or ribbons for wear for uniformities sake.
Your recruiter is mistaken as to the wear of medals being reserved for officers and only being worn on the mess dress uniform piece.

I just did a quick review of Army orders. From what I found, you are authorized to wear the full medals on your uniform and I heartily recommend you do so. This is a day for honoring those who have passed on while in the service. What better honor can you pay to them than putting out the extra effort to present yourself in the highest fashion.